1. Field of the Invention
The present invention relates generally to electrical switching apparatus and, more particularly, to a draw-out circuit breaker assembly including a door interlock assembly for controlling access to a draw-out circuit breaker housed within an enclosure having a door. The invention also relates to a door interlock assembly.
2. Background Information
Electrical switching apparatus used in power distribution systems are often mounted within a housing assembly either individually or in combination with other switchgear (e.g., without limitation, circuit switching devices and circuit interrupters such as circuit breakers, contactors, motor starters, motor controllers and other load controllers).
Some electrical switching apparatus, for example, low voltage circuit breakers, can be quite large. In order to facilitate movement (e.g., installation; removal; maintenance), a sizeable circuit breaker is commonly coupled to rollers which permit such circuit breaker to be drawn out of the housing assembly. Accordingly, such circuit breakers are commonly known in the art as “draw-out” circuit breakers.
The enclosure which houses the draw-out circuit breaker generally includes side walls, a top, a bottom, a door and the draw-out mechanism. The draw-out mechanism includes, for example, a combination of rails and rollers on one or both of the side walls of the enclosure and the sides of the circuit breaker. See, e.g., U.S. Pat. Nos. 4,002,864; 4,002,865; 4,017,698; 4,728,757; and 6,563,062. The circuit breaker may be further supported within a draw-out frame, commonly known in the art as a cassette or chassis. See, e.g., U.S. Pat. No. 6,031,192.
To prevent injury (e.g., electrical shock), it is often desirable to limit access to energized electrical equipment within the enclosure. For example, in order to provide access and perform maintenance on a draw-out circuit breaker, the circuit breaker must be drawn out of the enclosure. Improper maintenance procedures (e.g., failure to first de-energize or turn the circuit breaker OFF) could cause serious injury to maintenance personnel and damage to the electrical equipment. Accordingly, known prior art draw-out circuit breaker assemblies have incorporated a variety of mechanisms in an attempt to prevent access to electrical components housed within the enclosure until the circuit breaker has been disconnected from the power source. Each of these mechanisms has its own unique set of disadvantages.
For example, U.S. Pat. No. 4,002,865 discloses a draw-out type circuit interrupter with an interlocked mechanism. The circuit interrupter is housed in an enclosure having a door with a window through which to view a position indicator. A levering mechanism operates the draw-out unit between a connected position, wherein the circuit interrupter is fully within the enclosure, and a disconnected position, wherein the circuit interrupter is drawn out of the enclosure. However, the circuit interrupter does not include a mechanism for locking the door of the enclosure. Instead it provides an elaborate, complex barrier and levering mechanism to control movement of the draw-out unit.
U.S. Pat. No. 4,728,757 discloses an interlock assembly for preventing access to the draw-out mechanism of a draw-out circuit breaker which is in the closed (i.e. ON) position. The front cover of the circuit breaker includes a sliding door movable between a first position, wherein access is provided to a driver shaft assembly for operation of the draw-out mechanism, and a second position, wherein such access is prevented. A plunger and slider mechanism is operably associated with the access door to prevent the door from being moved from the second position to the first position unless the contacts within the circuit breaker are open (i.e., the circuit breaker is OFF). However, the interlock mechanism requires numerous complex, separate parts and modification to the enclosure front cover in order to provide the disclosed access door.
U.S. Pat. No. 6,388,868 discloses an automatically operating interlocking assembly requiring an electrical cabinet to be closed before connection of the equipment therein. The interlock is used to control access of the draw-out assembly. Such access involves inserting a tool through an opening in the cabinet door. The interlock includes a spring-biased locking plate which reciprocates between a locked position wherein the locking plate blocks the opening and resists engagement of the draw-out assembly, and an unlocked position wherein the locking plate permits access to and movement of the draw-out assembly. The locking plate is spring-biased towards its locked position. When the cabinet door is closed, a tab on the door engages a slot in the locking plate, thereby pushing the locking plate from its locked position to its unlocked position. Conversely, opening the door causes the locking plate to move, under spring pressure, from its unlocked position to its locked position. Therefore, access to the drive for moving the draw-out assembly is precluded unless the cabinet door is closed.
U.S. Pat. No. 6,445,570 discloses an automatic interlock assembly including a spring-biased brake bar controlling the position of a brake. The brake bar is biased so that it pulls the brake against one of the wheels of the draw-out circuit breaker, thereby preventing movement of the circuit breaker towards the rear, connected (i.e., ON) position. Closing the door pushes the brake bar, pivoting it away from the wheel.
U.S. Pat. No. 6,545,859 discloses another automatically operating interlock assembly for electrical cabinets housing draw-out circuit breakers. The interlock assembly includes a locking bar pivotally secured to a cam at one end and selectively engaging the cabinet's door at the other end. The cam is pivotally secured adjacent to the draw-out assembly and is spring biased in order to hold the locking bar against a locking tab on the door. When the draw-out assembly moves forward, disconnecting the electrical contacts, it contacts the cam rotating the cam so that the locking bar is retracted from the door permitting it to be opened. Similarly, when the draw-out assembly is fully engaged within the cabinet, the cam is permitted to rotate the locking bar back into engagement with the cabinet door.
Operation of each of the foregoing mechanisms relies upon the position of the draw-out circuit breaker with respect to the enclosure. There is a need, therefore, for a door interlock assembly capable of functioning independently from the position of the draw-out circuit breaker with respect to the enclosure in which it is housed.
There is, therefore, room for improvement in draw-out circuit breakers and door locking mechanisms therefor.